Coupling of in-situ Sm-Nd Systematics and U-Pb dating of monazite and allanite with applications to crustal evolution studies

Abstract

Here we demonstrate the capabilities of LA-MC-ICPMS Sm-Nd-isotope measurements applied to two of the most common LREE-enriched minerals in the Earth's crust: monazite and allanite. We show that high precision measurements of143Nd/144Nd comparable to solution-based MC-ICPMS measurements (< 0.5 epsilon units) can be achieved for laser spot sizes ranging from 16 to 50 μm. At this scale, sub-grain domains previously dated using in-situ U-Th-Pb geochronology (e.g., ion-microprobe) can be characterized with in-situ Nd-isotopic and Sm/Nd elemental measurement to determine robust initial143Nd/144Nd ratios and investigate grain-scale isotopic heterogeneities. In addition, the LREE-enriched pattern and Eu* anomaly can be simultaneously determined from in-situ Ce, Nd Sm, Eu, and Gd concentration measurements. This approach is applied here to granulite-facies metamorphic rocks previously dated in-situ using an ion-microprobe. It is shown that precise initial143Nd/144Nd ratios and LREE concentrations measured in monazite and allanite grains by LA-MC-ICPMS on grain mounts and in-situ in polished thin sections can provide constraints on isotopic inheritance, and an understanding of the origin of compositional domains (e.g., core-overgrowth features) within a textural context. Differences between monazite U-Pb and garnet-whole-rock Sm-Nd isochron ages can also be used to estimate crustal cooling rates, thereby elucidating the timescales of crustal residence of high-grade terrains.